Gene therapy has been considered effective means for treating intractable diseases, and 600 or more gene therapy protocols have already been proposed in different countries (mainly in Europe and the United States).
As has been known, vectors which are currently employed in gene therapy include adenovirus vectors and retrovirus vectors and so on. However, adenoviruses have been shown to cause severe inflammation in the liver or the brain in clinical studies, whereas retroviruses raise problems including insertion mutation resulting from random integration into chromosomal DNA, and induction of cancer associated therewith.
Meanwhile, adeno-associated virus (hereinafter may be abbreviated as “AAV”) vectors have the following characteristics: 1) being nonpathogenic, 2) having low immunoreactivity, 3) capable of transferring a gene into nondividing cells, and 4) enabling long-term transgene expression. Particularly, AAV vectors exhibit excellent safety as compared with, for example, the aforementioned retrovirus vectors or adenovirus vectors. In recent years, vectors based on various AAV serotypes (e.g., AAV serotypes 1 to 5) have been developed. Such AAV serotype vectors enable gene expression in various tissues on the basis of the difference in infection targets between these AAV serotypes. By virtue of the aforementioned characteristics, AAV vectors have been considered promising vectors for gene therapy. For example, conventionally, AAV vectors have generally been employed for protein replacement therapy through intramuscular administration. However, AAV vectors, which have low gene transfer efficiency (particularly in the case of gene transfer into cancer cells), have become of less interest in cancer therapy, which requires that a therapeutic gene be transferred into all the cells.
As has been reported, when an adenovirus vector is employed, expression an adenovirus receptor (integrin or CAR (coxsackievirus adenovirus receptor)) is increased by a histone deacetylase inhibitor, and transgene expression is enhanced. Meanwhile, it has been reported that when an AAV vector is employed, a histone deacetylase inhibitor reactivates transgene expression in cells which have been infected and then subcultured for a long period of time, in which the genome is integrated and is in a non-expression state (see, for example, Chen, et al., Proceedings of the National Academy of Sciences USA, Vol. 94, pp. 5798-5803 (1997)).
In gene therapy employing an AAV vector, a effective target is not reactivation of cells which are in a non-expression state, but the efficiency of the AAV vector-mediated gene transfer into cells (in an expression state). However, studies on such gene transfer efficiency have not yet been conducted.